1. Field of the Invention
An aspect of the present invention relates to an ion-conductive composite membrane and a method of manufacturing the same, and more particularly, to an ion-conductive composite membrane that effectively prevents crossover of liquid fuel without the reduction of ion conductivity in a fuel cell and a method of manufacturing the same.
2. Description of the Related Art
A group of fuel cells form an electrochemical apparatus in which chemical reaction energy between oxygen and hydrogen contained in a hydrocarbon-based material, such as methanol, ethanol, or natural gas, is directly converted into electrical energy. Since the energy conversion process of fuel cells is very efficient and environmentally friendly, various fuel cells have been researched.
Fuel cells can be categorized into phosphoric acid type fuel cells (PAFC), molten carbonate type fuel cells (MCFC), solid oxide type fuel cells (SOFC), polymer electrolyte membrane fuel cells (PEMFC), alkali type fuel cells (AFC), and the like, according to the electrolyte that is used. These fuel cells operate based on the same principle, but have different fuels, different operating temperatures, different catalysts, different electrolytes, etc. The PEMFC is effective for use in small stationary electric power plants and transportation systems because the PEMFC has several advantages such as low temperature operation, high output density, rapid start-up, and quick response for the change of output requirement.
The main portion of a PEMFC is a membrane electrode assembly (MEA). A MEA typically includes a polymer electrolyte membrane and two electrodes attached to opposite sides of the polymer electrolyte membrane and respectively acting as a cathode and anode.
The polymer electrolyte membrane functions as a separation membrane preventing the direct contact of oxidizers and reducing agents, an electric insulation between the two electrodes, and a proton conducting medium. Accordingly, the polymer electrolyte membrane is required to have high proton conductivity, high electric insulation, low reactant permeability, high thermal, chemical, and mechanical stability in operating conditions of fuel cells, and low costs.
To satisfy these requirements, various polymer electrolyte membranes have been developed. Perfluoropolysulfonic acid membranes such as a NAFION membrane are widely used because of their good durability and performance. However, sufficient moisture should be supplied to properly operate the NAFION membrane and the operating temperature should be lower than 80° C. to prevent moisture from being lost. In addition, carbon-carbon bonds in a backbone in the NAFION membrane may be attacked by oxygen (O2) making the bonds unstable in operating conditions of fuel cells.
In the case of DMFCs, a methanol solution is supplied to an anode as fuel, and thus some of the unreacted methanol permeates into a polymer electrolyte membrane of the DMFC. The permeation of the methanol into the polymer electrolyte membrane causes the electrolyte membrane to swell and the methanol diffuses to a cathode catalyst layer. This is referred to as ‘methanol crossover’. Accordingly, since the methanol is directly oxidized at a cathode at which electrochemical reduction of protons and oxygen occurs, the potential of the cathode decreases significantly thereby degrading the performance of the fuel cells.
Such a problem commonly occurs in other fuel cells using liquid fuel containing polar organic fuel.
Prevention of the crossover of polar organic liquid fuel such as methanol or ethanol has been actively researched. For example, inorganic fillers were dispersed in a polymer electrolyte matrix to obtain a membrane (U.S. Pat. Nos. 5,919,583 and 5,849,428) or an inorganic cation-exchange material was mixed with NAFION to form a organic/inorganic composite membrane (U.S. Pat. No. 6,630,265). For this purpose, the development of nano composite materials and the formation of an exfoliated layer of an inorganic material in polymers have been actively researched.
In the development of the nano composite materials, an inorganic material having proton conductivity is obtained from clay and exfoliated using polymers, or polymers are intercalated into gaps. This approach has brought about many improvements, but has not completely prevented the crossover of the polar organic liquid fuel.
An exfoliated layer of an inorganic material is formed by adding ion conductivity to an exfoliated inorganic material and coating the same on a base material. However, this method cannot completely prevent the crossover of the polar organic liquid fuel, and essentially requires the base material.